ABSTRACT: The use
of microbes and microbial products as bioherbicides has been studied for
several decades, and combinations of bioherbicides and herbicides have been
examined to discover possible synergistic interactions to improve weed control
efficacy. Bioassays were conducted to assess possible interactions of the
herbicide glufosinate [2-amino-4-(hydroxymethylphosphinyl) butanoic acid] and Colletotrichum truncatum (CT),
a fungal bioherbicide to control hemp sesbania (Sesbania exaltata)].
Glufosinate acts as a glutamine synthetase (GS) inhibitor that causes elevated
ammonia levels, but the mode of action of CT is unknown. GS has also been
implicated in plant defense in certain plant-pathogen interactions. The effects
of spray applications of glufosinate (1.0 mM) orbioherbicide (8.0 × 104 conidia ml-1),
applied alone or in combination were monitored (88 h time-course) on seedling
growth, GS activity and ammonia levels in hypocotyl tissues under controlled
environmental conditions. Growth (elongation and fresh weight) and extractable
GS activity were inhibited in tissues by glufosinate and glufosinate plus CT
treatments as early as 16 h, but CT treatment did not cause substantial growth
reduction or GS inhibition until after ~40 h. Generally, ammonia levels in hemp
sesbania tissues under these various treatments were inversely correlated with
GS activity. Localization of hemp sesbania GS activity on electrophoretic gels
indicated a lack of activity after 30 h in glufosinate and glufosinate plus
CT-treated tissue. Untreated control tissues contained much lower ammonia
levels at 24, 64, and 88
h after treatment than treatments with CT, glufosinate or their combination. CT
alone caused elevated ammonia levels only after 64 - 88 h. Glufosinate
incorporated in agar at 0.25 mM to 2.0 mM, caused a 10% - 45% reduction of CT colony
radial growth, compared to fungal growth on agar without glufosinate, and the
herbicide also inhibited sporulation of CT. Although no synergistic
interactions were found in the combinations of CT and glufosinate at the
concentrations used, further insight on the biochemical action of CT and its
interactions with this herbicide on hemp sesbania was achieved.